The present invention relates to magnetic seal devices which form seals between concentric components which rotate relative to each other, and, more particularly, to a magnetic liquid seal device which forms a seal in apparatus having an outer rotating component, such as a motor having an outer rotor which rotates around an inner shaft.
There are many instances where gaps between equipment components which rotate with respect to each other must be sealed. For example, in the case of magnetic data disc storage equipment used in data processing applications, it is important to hermetically seal the rotating equipment component in a manner such that vapors generated by gas emitting lubricants cannot enter into the area of the data discs. Such seals must be of high reliability, and, additionally have very low friction moments.
Magnetic liquid seal devices which form seals between rotating equipment components are known. European Patent Specification (EPS) No. 79 30 27 86.3 (Publication No. 0 012 556 B1) discloses a magnetic fluid seal constituted by a disc-like magnet magnetized in an axial direction positioned between the rotating components and fixed to one of them, and magnetic liquid held around a peripheral edge of the disc-like magnet by a magnetic field which passes from the pole on one face of the disc-like magnet through the magnetic liquid and back to the pole on the opposite face of the disc-like magnet to form the seal. The magnetic liquid is thus held in position exclusively under the effect of the magnetic flux. The above-mentioned EPS discloses embodiments in FIGS. 3, 5 and 6 wherein the sealing gap is formed between the inner periphery of the disc-like magnet and the rotating inner shaft. In another embodiment illustrated in FIG. 4 of the EPS, two sealing gaps are formed at the inner and outer peripheries of the disc-like magnet so that the disc-like magnet is adapted to rotate relative to both the inner shaft and an outer housing. This sealing arrangement is advanageous in that it is of relatively low cost construction and does not require much room. On the other hand, it is not entirely satisfactory in that the magnetic flux field will have a relatively low intensity so that its reliability is diminished with larger differences in pressure across the seal and/or at higher rotational speeds.
These disadvantages are eliminated by another known magnetic fluid seal arrangement wherein additional disc-like magnets are utilized to achieve greater magnetic flux field intensities in the sealing gap. However, this sealing arrangement has the serious disadvantage that the shaft to be sealed must be formed of a nonmagnetic material. After the shaft has been assembled, the seal gap can no longer be filled with the magnetic liquid. When the shaft is formed of ferromagnetic material, the magnetic liquid would be wiped off during assembly. A further disadvantage of the sealing arrangement is that it requires considerable room for installation due to the use of the additional disc-like magnets.